The present invention relates to digital video conversion systems, in particular, a system for use with multiple television sets at the home of a single subscriber.
Subscription and pay-per-view video services are delivered to a subscriber's premises via both satellite and cable transmission systems. In cable systems, 6 MHZ analog video and/or digital QAM channels are frequency division multiplexed (FDM) across the 500-1000 MHZ cable bandwidth. In typical satellite systems, several compressed digital video programs are time division multiplexed (TDM) into a single 10-40 Mbit/s QPSK or BPSK modulated C- or K- band carrier. Several carriers may be available to subscribers over several transponders in the satellite.
In order to select and view a program, the received broadcast signal must be tuned, demodulated, and decompressed. Additionally, in the case of premium or pay-per-view programming, authorization must be obtained for the digital data to be decrypted. In traditional systems, these functions are placed in a set-top converter box, which RF modulates the decompressed video for viewing on channel 3 or 4 of a standard television.
The tuning and demodulation functions of the conversion process for cable and satellite systems differ because of their transmission methods. The decompression and RF modulation of the digital video data are similar to both systems. In cable systems, the appropriate 6 MHZ channel is tuned by mixing the received FDM signal to bring the desired channel to an intermediate frequency (IF). The IF QAM signal is then demodulated into the compressed digital video data. For satellite systems, the set-top converter box does not receive the K- or C- band carrier directly, but is fed an amplified signal translated to the L-band. The converter box then tunes the desired channel to an IF frequency. In both systems, the digital video data is decompressed according to pre-defined algorithms (e.g., as an MPEG-2 program), then is modulated as an analog signal for viewing on a television set or monitor.
Because of advantages of digital video broadcast systems over analog systems in the quality, density, and versatility of services, digital systems are projected to soon replace analog transmission systems. However, by their nature, digital set-top converter boxes are more complex and expensive than analog set-tops. Additionally, digital video systems currently require one set-top box for each independently operating television, whereas many televisions are simply split off the cable drop to a household subscribed to an analog system. A television can usually be directly split off an analog system because many sets come equipped with "cable-ready" tuners. "Cable-ready" tuners are relatively inexpensive since they are mere extensions of terrestrial broadcast tuners. Further, premium channels are often authorized by simply removing a notch filter on the cable drop entering the premises of an analog system subscriber. Thus, although digital video transmission systems offer more and better services than analog systems, they suffer a higher cost per television set for a given subscriber.
Controlling the ability to view programming by only authorized subscribers or purchasers is of primary importance in a satellite or cable broadcast system. Unlike free terrestrial broadcasts which are paid by advertising sponsors, satellite and cable systems charge viewers directly for programming. Many access control systems have been deployed and were subsequently defeated. Because of arguments that any access control system can eventually be dissected and defeated, the state-of-the-art system features a "replaceable security" element such as a "smartcard". The smartcard is credit-card sized, and contains an integrated circuit chip for authorizing a unique subscriber. The smartcard is inserted into the set-top box to decrypt premium programming. A smartcard may be used on a pre-paid basis, or can be inexpensively replaced in the event the existing access control system is defeated.
In addition to the cost disadvantage of requiring an individual set-top box for each television in a household, problems also arise in managing access control for a household. For example, additional televisions in the home of an analog cable subscriber are often charged only a nominal fee, based on a multiple-license discount. However, in a digital transmission system with individual set-top boxes per television, a system operator could not offer such a discount since the smartcards could be moved to other homes. At the time of this invention, this necessity to pay for each individually operating set in a household remains a disadvantage of digital systems compared to analog systems.